If I listened to everybody who told me I wouldn't be able to do what I do today I'd still be working in a hot sweaty factory for near minimum wage.
Almost everybody rolled their eyes at me and cracked jokes behind my back.
Now, 6 years later, they ask me how I did it.
Lots and lots and determination. Lots of trial and error. Lots of sacrifice of the things I'd rather be doing (fishing, traveling, going out with friends...) in order to learn and become proficient at what I do.
You gotta start somewhere. It's not like he's saying he's going to become the next top sign designer.... he wants to start with yard signs. It's a starting point. He knows he's not going to get rich doing it. He knows it's the bottom of the barrel.
No offense to some of the people here, but several of you don't even do your own designs, printing, or fabrication... you outsource it all and the customer pays the price for your "knowledge and skill".
You gotta start somewhere. An idea seems like a good start to me. The rest will follow if it's meant to be.